The present invention relates to a multicolor thermosensitive image transfer sheet which can provide a good multicolor image transfer with imagewise application of heat by a thermal head, laser, or the like, and to a recording method by using the multicolor thermosensitive image transfer sheet.
Conventionally, there is known a process of obtaining multicolored images by using (a) an image transfer medium for each color comprising a support and a thermosensitive image transfer ink layer formed thereon, comprising a coloring agent and a thermofusible material, or (b) an image transfer medium for each color comprising a support and a thermofusible image transfer ink layer formed thereon, comprising a fine porous structure made of a resin, containing therein a thermofusible material which is solid at room temperature, but can be melt when heated, and a binder agent, by (i) disposing the image transfer layers in a plurality of colors side by side, (ii) applying heat to the image transfer layers by a thermal head, a laser or the like, and (iii) transferring the image transfer layers imagewise successively to a receiving sheet in such a manner the colors may be built up.
In this case, if the same material is used for the thermofusible material for each color image transfer medium, the wetting characteristics are exactly the same, so that the image transfer ink layers, subsequently transferred to a receiving sheet, may well adhere to each other when transferred in a superimposing manner to a receiving sheet. Therefore, when the colors are built up, there is no problem in making each ink layer adhere to the ink layer which comes into contact therewith as it is applied. However, if there are many solid images, the amount of the image transfer layer components transferred from each color image transfer medium tends to become excessive. The result is that the hue of the topmost ink layer becomes dark, and a defect occurs in which the color balance of the image is degraded.
Recently it has been proposed to use an appropriate thermofusible material in each color image transfer medium, without using the same thermofusible material as mentioned above, from the aspect of the balance of the formulations of the ink layers, that is, from the aspect of the optimum match of the coloring agent and the thermofusible material in each color image transfer medium. The result is that such a defect occurs that the wetting characteristics of each color image transfer medium are different so that the mutual adhesion of the thermosensitive image transfer layers becomes poor, when superimposed on a receiving sheet.
A previous proposal by the inventors of the present invention is (i) a multicolor thermosensitive image transfer sheet comprising a plurality of thermofusible ink layers, each ink layer containing therein a thermofusible material having a different critical surface tension for each color, (ii) and a multicolor thermosensitive image transfer recording method by using the multicolor thermosensitive image transfer sheet in a sequence from a high critical surface tension of the thermofusible material to a low critical surface tension thereof in the thermofusible ink layers so that the colors are built up. However, if a different coloring agent is used in each thermofusible ink layer or a surfactant is employed in a different amount in each ink layer, the critical surface tension of each ink layer considerably differs when transferred to a receiving sheet. Therefore, the improvement of the above-mentioned color build up cannot be completely attained by merely adjusting the critical surface tension of the thermofusible material for use in each ink layer.